SUMO-1 is a ubiquitin-like protein. SUMO-1 can be covalently conjugated to other proteins through an isopeptide linkage in a manner similar to ubiquitin. A number of SUMO-1 conjugation substrates have been reported in vertebrates to date; these include RanGAP1, RanBP2, PML, Sp100, IkBa, HIPK2, p53, topoisomerase I, c-Jun and the Werner's Syndrome gene product. Western blotting analysis suggests that the reported SUMO-1 substrates are a subset of the total number of cellular proteins that are conjugated to SUMO-1 in vivo. The SUMO-1 conjugation pathway utilizes enzymes that show both sequence similarity and structural conservation with ubiquitin activating (E1) and conjugating (E2) enzymes. Moreover, biochemical analysis suggests that ubiquitin and SUMO-1 enzymes carry out their functions using similar enzymatic mechanisms. However, none of the enzymes characterized to date act in both the ubiquitin and SUMO-1 conjugation pathways. We are studying the SUMO-1 pathway in vertebrates. We are particularly interested in the regulation of this pathway during the cell cycle and its role in controlling the Ran GTPase activating protein RanGAP1 through conjugation. Our goals are to understand this pathway and its regulation at a molecular level, to determine whether and how it regulates mitosis in vertebrates. Toward this goal, we have analyzed the localization, abundance and behavior of enzymes involved in SUMO-1 conjugation and deconjugation, as well as the disruption of this pathway in disease states (acute promyelocytic leukemia) and through the action of viral proteins (HSV-1 ICP0).